Improve handling of period tokens and comments

The variable last_hash_sym was removed, since it was not used after the
improvement.
Add more 'receive' tests
Refactor 'receive-after' handling

We don't need a separate last_receive variable, we can use last_token to store
whether the last token was receive or not.
Refactor ErlangAnalyzeLine to return last_token
Add debugging facilities to helper.vim
Refactor 'receive-after' to use proper tokenization